ON THE COLOURS OF THICK PLATES. 189
P. Let P' be the particle nearest to the former position of P; and, to make a supposition as favourable as possible to interference, let P' be the very particle P moved a little along the surface without rotation. Although the interval of retardation R of the two streams diffracted by P in its first position, and reaching a given point of space, is sensibly the same as the interval of retardation of the two streams diffracted by P in its second position, and reaching the same point, yet this interval would be changed altogether were the transference of position to take place during the interval of time which elapses between the departure of the wave from P and its return after reflexion, as may very readily be seen. The amount, too, by which the interval of retardation would be changed would vary in an irregular manner from one particle to another, and therefore no regular interference would take place. Now the purely ideal case just considered is precisely analogous to the case of actual experiment when a luminous point is viewed through a plate of glass with both faces dimmed, since the particles ou one face have no relation to those on the other. We ought not therefore in such a case as this to expect to see rings or bands.
29. According to the formula (35), the angular breadth of one of the bands formed by a plane mirror becomes considerable when i becomes nearly equal to 90°, so that, apparently, bands ought to be visible at a large angle of incidence. But if the courses of the two streams scattered by the same set of particles be traced, it will be found that they are so widely separated that, for various reasons, no regular interference can be expected to take place. Accordingly, the bands are not seen at a large angle of incidence.
30. In the preceding sections I have spoken of the light by which the rings are formed as having boon scattered at, the dimmed surface. And so it. really is, if by that term we merely understand deflected from the. course it would have followed according1 to the regular law of refraction. But according to the explanation given in the preceding article the light, is not scattered, in the strict sense of the term, but regularly diffracted. Scattered light is, strictly speaking, such as that by which objects are commonly seen, or again, such as that which is transmitted through white paper and similar substances. The preceding view of the nature